Ok. I am going to bite on this one. I am certified through the Spine Research Institute of San Diego for Whiplash and brain injury traumatology, this includes collision reconstruction. I am able to testify and an expert witness in legal proceedings as to crash mechanics and injury bio-mechanics.
That test they did was a 40% offset crash . The reason they do the crash test is most frontal collisions happen like that. Frontal collision generally do not hit 100% head on. In a direct head on the outcomes would have still favored the Malibu.
While mass of vehicles is an important factor in crash worthiness, size is not the whole story. Vehicular restitution, ride down, chassis stiffness in an accident is very important as it relate to the delta v of the occupants.
The 59 Belair has a standard frame that is terrible in an accident. As you can see in the film, the Belair deforms dramatically upon impact. The passenger compartment is compromised severely. The a-pillar crushed like an egg. The A-pillar rigidity is crucial for occupant safety since its collapse will allow for the deformation of the dash area. Look at crash video of a 2002 Kia Sedona and you will see another prime example of poor occupant cage design.
Also look at the rebound of the two cars. the Malibu stayed semi flat as it rebounds away from the accident the Belair's rear end goes drastically airborne. Ride down from the accident is very important in the injuries sustained to the occupants. The longer the ride down the less force sustained to the occupants.
The ride down in a 59 Bel air is very poor.
Even though the larger Belair has the energy advantage, the occupant in the Malibu will have a decent chance at survival of a side impact crash due to the side impact airbags and other safety features. In addition not all side impact crashes are the same. You have near side and far side impacts. The out comes and injuries of the two types are drastically different.